1. Field of the Invention
The present invention relates to an image generating apparatus, and more particularly, it relates to an image generating apparatus comprising a thermal head.
2. Description of the Background Art
An image generating apparatus comprising a thermal head is known in general. This conventional image generating apparatus prints images on a paper or the like by applying energy to a heating element so that the heating element generates heat. In general, the aforementioned image generating apparatus controls a thermal head by detecting the temperature thereof or the ambient temperature around the thermal head. For example, Japanese Patent Laying-Open Nos. 5-155059 (1993), 6-297748 (1994), 6-238939 (1994) and 10-217529 (1998) disclose such image generating apparatuses.
Thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 5-155059 is so formed as to detect the ambient temperature of the image generating apparatus with a thermistor (temperature sensor) for controlling an electrification type for a thermal head.
Thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 6-297748 receives temperature information from a head temperature detecting portion and a printing ambient temperature (ambient temperature) detecting portion and controls a pulse width of a thermal head with a fuzzy inference portion on the basis of a fuzzy inference, in order to keep a printing concentration constant. The fuzzy inference portion is so formed as to control the pulse width to three levels by reducing the pulse width if the temperature of the thermal head is higher than a prescribed thermal head temperature, setting the pulse width to an intermediate level if the temperature of the thermal head is substantially identical to the prescribed thermal head temperature and increasing the pulse width if the temperature of the thermal head is lower than the prescribed thermal head temperature. Further, the fuzzy inference portion is so formed as to control the pulse width to two levels by slightly reducing the pulse width if the printing ambient temperature is higher than a prescribed printing ambient temperature (ambient temperature) and slightly increasing the pulse width if the printing ambient temperature is lower than the prescribed printing ambient temperature. The fuzzy inference portion can suppress dispersion of the printing concentration by selecting any of the five pulse width levels in response to the temperature of the thermal head and the printing ambient temperature (ambient temperature).
Thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 6-238939 reduces influence exerted by the ambient temperature by detecting the ambient temperature with ambient temperature detection means and controlling an applied voltage and/or an electrification time to three printing conditions by applying a prescribed voltage for a prescribed electrification time if the ambient temperature is within a previously set reference temperature range, reducing the applied voltage below the voltage applied when the ambient temperature is within the reference temperature range if the ambient temperature is higher than the reference temperature range and increasing the electrification time from that employed when the ambient temperature is within the reference temperature range if the ambient temperature is lower than the reference temperature range.
Thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 10-217529 is provided with a temperature measuring apparatus for acquiring the temperature of a thermal head, for determining unprintableness if the temperature of the thermal head exceeds a reference value and interrupting printing until the temperature of the thermal head is reduced to a printing start temperature allowing printing initiation. According to Japanese Patent Laying-Open No. 10-217529, the printing start temperature allowing printing initiation, calculated on the basis of the quantity of printing, is increased if the quantity of printing is small, so that the printing can be returned in an early stage.
However, the thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 5-155059 controls the electrification time in consideration of only the ambient temperature of the image generating apparatus, regardless of temperature rise resulting from continuous service of the thermal head. When the thermal head is continuously used for continuously printing the same image, therefore, it is difficult to supply a proper heat quantity to the thermal head. Therefore, printing quality is disadvantageously remarkably dispersed in an initial stage of continuously printing the same image and after printing this image on a constant number of papers.
In the thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 6-297748 controlling the printing concentration by selecting the pulse width from the two levels in relation to the printing ambient temperature (ambient temperature), the printing concentration cannot be correctly controlled to levels beyond the two levels if the ambient temperature fluctuates. If the ambient temperature fluctuates when the image generating apparatus continuously prints the same image, therefore, it is disadvantageously difficult to sufficiently reduce dispersion of the printing quality between an initial stage of continuously printing the same image and after printing this image on a constant number of papers.
In the thermal head control means described in the aforementioned Japanese Patent Laying-Open No. 6-238939 controlling the applied voltage and/or the electrification time to the three printing conditions on the basis of comparison between the ambient temperature and the reference temperature range, the applied voltage and/or the electrification time cannot be correctly controlled to printing conditions beyond the three conditions. If the ambient temperature fluctuates when the image generating apparatus continuously prints the same image, therefore, it is disadvantageously difficult to sufficiently reduce dispersion of the printing quality between an initial stage of continuously printing the same image and after printing this image on a constant number of papers.
The aforementioned Japanese Patent Laying-Open No. 10-217529, disclosing the thermal head control means calculating the printing start temperature allowing printing initiation from the quantity of printing, discloses no technique of correcting the printing concentration on the basis of the ambient temperature or the like. If the ambient temperature fluctuates when the image generating apparatus continuously prints the same image, therefore, the printing quality is disadvantageously increased between an initial stage of continuously printing the same image and after printing this image on a constant number of papers.